Reactive medium for purifying fluids

ABSTRACT

A method for forming a reactive medium for removing homogenous and heterogeneous impurities from a fluid is provided which includes forming a uniform oxide layer on all exposed surfaces of a porous metal substrate, depositing at least one layer of carbon on substantially all of the oxide layer formed on the porous metal substrate, depositing a precursor metal species on the carbon layer, and heating the porous metal substrate containing the carbon layer to form active sites on the carbon layer, wherein the active sites include at least partially deoxygenated metal species chemically bonded to the carbon layer. Also provided is a reactive medium for purifying a fluid, the reactive medium comprising a porous inorganic substrate, a layer including carbon and an oxide layer between the porous inorganic substrate and the layer including carbon. The reactive medium further comprises active sites including at least partially deoxygenated metal species, the active sites being bonded to the layer including carbon.

This application is a divisional of U.S. application Ser. No. 08/976,118filed on Nov. 21, 1997, now U.S. Pat. No. 5,829,139 which is acontinuation of U.S. application Ser. No. 08/433,022 filed on May 3,1995 and now abandoned, both of which are incorporated by reference intheir entireties.

FIELD OF THE INVENTION

This invention relates to a filter assembly used for filtering fluids.More particularly the present invention is directed to a filter assemblyused in the filtration and ultrapurification of gases and to a method offiltering and purifying gases.

BACKGROUND OF THE INVENTION

Advances in all areas of modern technology have, to a large degree,depended on the modification and development of new materials and thepurification of substances used as both reagents and as materials, inthe presence of which various processes are conducted. The purificationof many such substances, typically liquids and gases, has required theremoval of impurities which are either heterogeneous (such as particlesand macromolecules) or homogeneous (such as dissolved substances).Typically, heterogeneous impurities are removed by filtration techniquesand devices in which the particles are physically retained by some sortof perforate or porous medium. Other methods and purification techniquesare typically chosen to remove homogeneous impurities. Many of thesetechniques involve the chemical modification and/or the affinity andattraction of the homogeneous impurity and the resulting removal of suchmaterial from the fluid.

In many areas of modern technology, the concentration of impuritiesabove several parts per million (ppm) cannot be tolerated and in certaintechnologies, such as in the manufacture of semiconductor devices, theconcentration of impurities in both the substances used as reagents aswell as other materials, in the presence of which the processes areconducted, can still be detrimental even at levels at or below severalparts per billion (ppb). For example, in many of the process gasesemployed in manufacturing semiconductor devices, impurities such asmoisture, oxygen and organic compounds in even trace amounts can beadsorbed on the semiconductor wafer, causing degradation of performance,reduced manufacturing yield and adverse reliability.

In such applications, to remove homogeneous impurities from processgases, various commercial purification techniques and purifiers whichinvolve physical adsorption and chemisorption of impurities orconversion of impurities to other forms which can be adsorbed on a solidsubstrate are employed. Most of these purification techniques employ apacked bed of particles or expanded materials. Examples of suchmaterials include various resins (e.g., Nanochem® resins) and variousalloys (e.g., Zr--V--Fe alloys). For purification by this technique, thegas stream is passed through these packed beds and the impurities reactwith the sorption material. While capable of removing impurities on theppm level, these purification materials often do not effectively filtertrace homogenous impurities, such as reactive gases, present at the ppblevel. Moreover, these packed beds tend to be ineffective when there isan abrupt surge in the impurity level due to the inefficiency of thepacked beds in effecting contact between impurity molecules and theresin or alloy of the bed material. In addition, because such materialsthemselves tend to generate heterogeneous impurities because ofmechanical motion and attrition of particles of the packed bed material,they have limited service lifetimes. Furthermore, these materials aretypically not reusable and often cannot be regenerated.

For the removal of particulate material, various porous ceramics havebeen employed. For example, U.K. Patent No. 2,201,355 to Dahlquist etal. employs a porous membrane for separating heterogeneous impuritiesfrom an aqueous medium. The porous membrane comprises an outer supportmatrix having through-passages and an inner layer lining thethrough-passages and deposited on the outer support. A polymer, metal orceramic is used as the support matrix. The inner layer is a matrix ofparticles of aluminum hydroxide, partially hydrated aluminum oxide,silicon dioxide or zirconium dioxide. French Patent No. 2,251,351describes a microporous ceramic filter that includes a microporousceramic support electrophoretically coated with an oxide of Al, Si, Mg,Ti, Cr, Ni, Zr or Fe. U.S. Pat. No. 3,288,615 to Estes et al. describesa ceramic filter body that includes a framework of one or moretectosilicates with a mineral species (e.g., aluminates and oxides)distributed throughout and filling the framework.

While these and other filtration and purification materials permitremoval of impurities with varying degrees of success from a gas streamduring operation, significant potential for introduction ofcontamination to the system occurs during start-up periods. This isparticularly true when the filtration or purification device is placedinto service in the process stream. Thus, referring to FIG. 1, whichillustrates in section a conventional filter device used in gas processstreams, a filter is incorporated within a housing through which the gasstream flows. More particularly, the device 1 includes a fluidpurification filter 3, often a reactive gas filter, disposed within thehousing 5 having a fluid inlet 7 and fluid outlet 9 formed as fluidconnectors at opposite ends of the housing. Such an arrangement requiresthat all of the gas pass through the fluid purification filter 3 beforereaching the outlet 9. Prior to use, caps, typically formed from a metalmaterial, are placed over the inlet and outlet connectors, 7 and 9, tokeep dust, other particulate contaminants, and in some instances,fluids, from entering the housing. Immediately prior to use, the capsare removed so that the housing may be inserted into the system. Deviceswith structures of the type shown in FIG. 1, however, allow for theintroduction of contaminants from a number of sources to the system.

One source of contamination originates within the housing 5 between thefilter 3 and the inlet 7 from air or other fluid present in or flowinginto the upstream portion of the housing through the inlet 7 when thedevice is installed in the process stream. In many instances, air,components of air or impurities in air, are detrimental to the processstream and the reactive gas filter is capable of removing theundesirable components or impurities present in the air which wouldadversely affect the process stream. Once the device is installed in theprocess stream and flow begins to pass from the process stream throughthe housing, the air initially present in or entering the housingthrough the inlet does not introduce further impurities to the processstream since the stream passes through the filter 3. It is, however,desirable to minimize the amount of air, contaminants, etc. entering thedevice since the filter 3 has a limited capacity to remove impurities.To minimize the amount of air entering the process stream at the inlet 7when the device is installed, a valve, such as a poppet valve, could beplaced within the inlet 7 and the housing pressurized with the gasemployed in the process stream while the outlet of the housing iscapped. However, while such an approach is effective in some instancesto flush a filter device such that it may be used immediately after thedevice is capped, in other instances there is the potential forintroduction of a second source of contamination into the processstream. Thus, depending upon the location and climatic conditions wherethe filter device is pressurized and the location and atmosphericpressure where the filter device is used, the gas in the downstream sideof the filter housing, between the filter 3 and the outlet 9, may bedisplaced by atmospheric air or other fluid in the vicinity of thehousing stream once the cap is removed from the outlet and the filterhousing is connected in the fluid stream. Although in some instances itmay be possible to flush the filter device with an inert gas or the gasemployed in the process stream at the time the device is placed intoservice, this requires one or more additional steps.

With current technologies, in addition to sources of contaminationintroduced from the ambient environment of the filtration/purificationassembly when the device or assembly is placed into service,contamination may be introduced by diffusion of atmosphericcontamination located in the vicinity of the inlet and particularly theoutlet of the housing. While the concentration of such contaminants maybe very small, in particular applications even trace amounts of suchcontaminants may be detrimental to the processes and products involved.

SUMMARY OF THE INVENTION

The present invention is directed to a filtration/purification assemblyfor use in fluid streams, preferably gas streams, which is capable ofremoving contaminants from the fluid stream and which avoidsintroduction of contaminants to the fluid stream and fluid assembly atthe time the filter assembly is placed in operation. Thefiltration/purification assembly of the present invention is capable ofreducing both homogeneous and heterogeneous contaminants toconcentrations in the ppb to parts per trillion (ppt) range. The presentinvention is also capable of minimizing or eliminating contaminationfrom entering the filtration/purification assembly, and hence the fluidstream, at the time the filtration/purification assembly is placed intoservice and start-up of the fluid stream is commenced. This results ingreater capacity and longer service life for the assembly.

The filtration/purification assembly of the present invention whichprovides such filtration and purification levels includes a housinghaving a fluid inlet and a fluid outlet. Disposed within the housingbetween the fluid inlet and fluid outlet, and extending across a fluidflow path provided therebetween is a fluid permeable, reactive filtermedium, preferably a gas permeable reactive medium. At both the fluidinlet and fluid outlet are disposed barrier members. As defined withrespect to the present invention, "barrier member" is a member whichprevents particulate and fluid contaminants from entering the inlet oroutlet, and thereby the housing. In some instances the member may beused to prevent entry of all fluids to the housing, except undercontrolled conditions. The barrier member may be a medium or membraneor, a device, such as a valve. It is preferred that at the fluid outletthe barrier member take the form of a medium, preferably a type of areactive medium, similar to the fluid permeable, reactive filter mediumdisposed intermediate the fluid inlet and fluid outlet and employed asthe main means of removing the bulk of contaminants entering thehousing. At the inlet, it is preferred that the barrier member be eithera valve, such as a poppet valve, or a medium, such as a reactive medium,also similar to the fluid permeable, reactive filter medium used in theinterior of the housing.

The reactive medium employed in the present invention includes a porousinorganic or organic substrate, preferably a metallic substrate havingexposed surfaces, and including at least one carbon layer deposited onsubstantially all of the exposed surfaces of the substrate. The carbonlayer is modified to present active sites which include at leastpartially or substantially deoxygenated metal species chemically bondedto the carbon layer. As used herein, the terms "rat least partiallydeoxygenated metal species" and "substantially deoxygenated metalspecies" refer to metals which have been reduced so as to be chemicallybound to less than the stoichiometric amount of oxygen.

The device according to the present invention is highly effective inremoving a variety of contaminants, both heterogeneous and homogeneous,from fluids, particularly gases. Accordingly, the purification system ofthe present invention may find application in a variety of processes.For example, the filter assemblies of this invention may be employed inthe removal of gaseous and particulate contaminants in gas or liquidsource streams used in electronics manufacture. Depending upon theselection of the partially deoxygenated metal species which forms partof the reactive medium, trace impurities such as oxygen, water, carbonmonoxide, carbon dioxide, methane and other hydrocarbons may be removedfrom a gas stream (e.g., a stream of an inert or noble gas such asnitrogen or argon). In addition to inert gas streams, the presentinvention may be employed to remove moisture from streams of reactivegases such as oxygen, silanes, hydrogen chloride and hydrogen bromide.

The device of the present invention is capable of reducing the totalconcentration of impurities, both heterogenous and homogeneous, to nomore than about 10 ppb, preferably to no more than about 1 ppb, and insome instances below 10 ppt. The device according to the presentinvention is capable of reducing, in an inert gas stream, theconcentration of water, oxygen or carbon monoxide to no more than about10 ppt, methane and other hydrocarbons to a concentration of no morethan about 10 ppt and preferably to no more than about 2 ppt and carbonmonoxide to no more than about 12 ppt.

As indicated above, in addition to purifying gases, once inserted into afluid stream, the filtration/purification assembly of the presentinvention is also able to remove impurities from ambient fluids,particularly gases, such as air, at the time the device is placed intoservice. This is true independent of such factors as whether the deviceis pressurized at the time of manufacture or prior to use, where thedevice was manufactured or what ambient conditions existed at the timeof manufacture or use. Thus, a barrier member located at the inlet ofthe assembly will minimize or substantially eliminate entry ofcontaminants to the housing. In most instances, there is little to nochance of contaminants being present in the gaseous volume in theportion of the housing between the housing inlet, or the upstreambarrier member, and the reactive filter medium, known as the "upstreamcapture volume". However, even if trace quantities of contaminants arepresent, once the device of the present invention is connected in lineto the fluid stream at both the inlet and outlet provided in thehousing, initiation of fluid flow will cause fluid in the upstreamcapture volume to pass through the reactive medium, thereby removing anyhomogeneous and heterogeneous contaminants in the upstream capturevolume. Likewise, particularly when the barrier member at the outlet endof the housing is formed from the same or a similar material as thereactive filter medium, contaminants present in the "downstream capturevolume" (the volume of gas in the portion of the housing defined by thewalls of the housing, the reactive medium and the outlet end of thehousing or barrier member located at the outlet) will be substantiallyretained by the reactive filter medium in the outlet when thefiltration/purification assembly is placed in a fluid stream. As aresult, the barrier members employed in the invention increase thecapacity and extend the service life.

In addition to being capable of reuse and long service life, thereactive filter device of the present invention is also capable of beingregenerated.

As suggested above, the present invention is also directed to a methodfor filtering and purifying fluids, particularly gases, to removeheterogeneous and homogeneous impurities to levels of several hundredppt and in some instances down to below the 10 ppt level. Such a methodinvolves placing the assembly in a fluid stream, such as a gas stream,and providing fluid communication to the inlet of the housing such thatthe fluid passes through the reactive medium which retains thecontaminants present in the fluid stream and provides purified effluent.

The present invention also relates to a method of preparing a reactivefiltration/purification medium and to a method of forming a reactivefiltration/purification assembly. The medium and the reactivefiltration/purification assembly itself produce higher purity fluidstreams and demonstrate an increased capacity to remove impuritiescompared to known systems. The method of forming the improved reactivemedium according to the present invention involves forming a uniformoxide layer on all exposed surfaces of a porous organic or inorganicsubstrate, preferably a metal substrate. Thereafter, at least one layerof carbon is deposited on substantially all of the oxide layer formed onthe porous metal substrate and afterwards a precursor metal species isdeposited on the carbon layer. Subsequently, the porous metal substratecontaining the carbon layer is heated to form active sites on the carbonlayer such that the active sites include at least partially deoxygenatedmetal species chemically bonded to the carbon layer. In forming thefiltration/purification assembly, the porous substrate is disposed in ahousing section prior to formation of the uniform oxide layer andsubsequent treatments.

The uniform oxide layer of the metal substrate is preferably formed byinitially contacting all of the exposed metal surfaces of the metalsubstrate with a reducing agent. Thereafter, the treated metal surfacesare contacted with an oxidizing agent at an elevated temperature.According to the preferred method of forming the reactive medium, theuniform oxide coating provided on the exposed metal surfaces of thesubstrate serves to initiate the deposition of carbon and to achieveimproved anchoring or bonding of the carbon to the substrate. Thisultimately results in a greater number of active sites on the medium.

The present invention further relates to a reactive medium for purifyingfluids. The reactive medium comprises a porous metal substrate, a layerincluding carbon, and an oxide layer between the porous inorganicsubstrate and the layer including carbon. The reactive medium furthercomprises active sites including at least partially deoxygenated metalspecies, the active sites being bonded to the layer including carbon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional gas filtration device.

FIG. 2 is a sectional view illustrating an embodiment of the presentinvention.

FIG. 3 is a sectional view illustrating another embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is illustrated in cross-sectionin FIG. 2. This embodiment includes a reactive filtration/purificationassembly 11 which incorporates a reactive filter medium 13 disposedwithin a housing 15 transverse to the fluid flow direction, as indicatedby an arrow in FIG. 2. The reactive filter medium 13 is so positionedand affixed within the housing 15 that all fluid entering the housingmust flow through the reactive filter medium. At one end of the housing15 is provided a fluid inlet 17 and at the opposite end of the housingis provided a fluid outlet 19. While the housing may have anyconfiguration, provided that the reactive filter medium extendscompletely across the fluid flow path in the housing between the fluidinlet 17 and the outlet 19, a preferred configuration is a cylindricalhousing 15, such as that illustrated in FIG. 2, in which a circularreactive filter medium 13 is placed centrally or coaxially within thehousing. Within the inlet 17 and in fluid communication with theinterior of the housing, particularly the upstream capture volume 25, isa first barrier member 21. Located within the outlet 19, preferably ator proximate the opening of the outlet at which effluent fluid emergesfrom the assembly, and in the fluid communication with the housing,particularly the downstream capture volume, is located a second barriermember 23.

The function of both the first barrier member 21 and the second barriermember 23 is to minimize or substantially eliminate entry of eitherheterogeneous or homogeneous contaminants to the filter housing 15 priorto placement of the assembly into service in the fluid stream, andthereby increase the capacity and service life of the assembly.Typically, such barrier member takes the form of either a valve or atype of reactive filter medium similar to that used to remove the bulkcontaminants from the fluid stream passing through the housing (i.e.,reactive filter medium 13). The embodiment illustrated in FIG. 2includes a valve, in this instance a one-way valve such as a poppetvalve at the inlet. A type of reactive filter medium is located at theoutlet. The embodiment illustrated in FIG. 3 includes a type of reactivefilter medium as the barrier member at both the inlet and outlet of thefiltration/purification assembly device.

Preferred as the barrier member 23 (and 23'), located at the downstreamopening of the outlet 19, is another or second reactive filter medium,similar in some respects (as discussed below) to the first or mainreactive filter medium 13.

The barrier member 21 (or 21'), provided at the inlet 17, is a poppetvalve in the embodiment of FIG. 2 (and a reactive filter medium in theembodiment illustrated in FIG. 3). Thus, in the embodiment illustratedin FIG. 3, the barrier members 21' and 23' are of the same type, i.e.reactive filter media which are similar to the fluid permeable, reactivefilter medium 13, located within the housing. It is also preferable inthe embodiment shown in FIG. 3, that the materials from which barriermembers 21' and 23' are made are the same. Depending on the particularapplication, the barrier members 21' and 23' may have the same ordifferent fluid flow characteristics as each other.

It is preferred to locate the barrier member 23 (or 23') in the fluidoutlet 19 proximate the downstream opening to minimize or substantiallyeliminate the volume between the tip of the outlet and the barriermember 23 (or 23') to prevent ambient gases from entering the housing 15and the fluid stream as the assembly is being inserted into the line. Itis also preferred, but is not as important as with the downstreambarrier member 23 (23'), to place the upstream barrier member 21 (or21') near the inlet 17 since any fluid flowing into the housing will befiltered and purified by the main reactive filter medium 13 beforereaching other parts of the fluid stream.

Other than those reactive filter media used as barrier members, thepurification system of the present invention has been described thus farand illustrated in FIGS. 2 and 3 as having a single layer or sheet ofreactive filter medium 13 fixedly positioned within the housing betweenthe inlet 17 and the outlet 19. While this embodiment is suitable formost applications, and is preferred because of ease of manufacturing andother reasons, there may be instances in which a plurality of sheets ofreactive filter medium, in spaced relationship to one another, may besubstituted for the single sheet reactive filter medium. Thus, in someinstances two or more sheets of reactive medium having the same ordifferent fluid flow or chemical composition characteristics, may bearranged in series within the body of the housing. In such instances,the overall pressure drop across the serially arranged sheets must meetthe requirements of the particular application in which the assembly isemployed.

In a similar manner, rather than the single layer or sheet of reactivemedium 21' employed as the barrier member in an embodiment of the typeillustrated in FIG. 3, a plurality of sheets, preferably two, ofreactive medium may be substituted for the single sheet illustrated.

In addition, upstream of the barrier member employed in the inlet,typically the reactive filter medium(s) 21', a filter element (notshown) may be provided to trap larger particulate material. Such filterelement may be formed from a material which is inert under coating,purification and filtration conditions and which will not contain orform volatile substances. The material should be of an appropriateporosity to trap particulate material slightly larger than would beretained by any reactive filter medium used in the device. Typicallyorganic polymeric and metal materials may be used in the upstreamfilter. The filter element may be a surface filter or a depth filter,appropriate to the situation. Such an embodiment is not expected to findwidespread application in the electronics industry, particularly in thesemiconductor industry, since the fluids employed therein may containsome particulate material but of relatively low proportions and of avery fine nature.

The preferred reactive filter media of the present invention are similarto those of the type described in U.S. Pat. No. 5,637,544, specificallyincorporated herein by reference. Such reactive filter media include atleast one substrate layer, preferably an inorganic substrate layer,having a plurality of pores therein, at least one layer of carbondeposited on each substrate layer and coating the pores thereof, and atleast one reactive layer of a metal in reduced form (i.e., a partiallyor substantially deoxygenated metal species) which is chemically bondedto the carbon layer and is capable of reacting with impurities in a gas.As indicated above, reactive media filters are different than most typesof existing filters in that in addition to being able to removeheterogeneous impurities, they are also capable of significantlyreducing homogeneous impurities by interacting with and removing traceimpurities.

When used as the main, fluid permeable, reactive filter medium 13,between the inlet and outlet of the housing, typically the mediumconstitutes a composite medium in which a plurality of coated substratelayers contact each other. The number of substrate layers employed forthe reactive filter medium 13 depends on factors such as pressure dropacross the reactive filter medium or matrix of media, particle removalefficiency and total contaminant removal capacity. Typically the numberdepends on the surface area and fluid flux necessary to achieve asatisfactory separation layer. When used as the barrier member 21', 23or 23', the number of layers is generally fewer than the number used inthe reactive filter medium 13 and is typically one to several layers.

With thinner reactive medium filters or barrier members, particularlythe latter, when the medium tends to be relatively non-self-supporting,it may be desirable to include a foraminous metal support within orsurrounding the barrier member or filter member. Typically this is avery open pore material, such as a metal mesh, particularly a stainlesssteel mesh. Suitable would be any open pore fibrous or powder materialor mesh in various configurations. Preferred is a K-mesh material, whichis a fine wire stainless steel mesh. Most preferred is a substrate suchas that available from Pall Corporation under the tradename of"ULTRAMET-L" which includes fine stainless steel fibers in a supportenvelope of a fine stainless steel mesh screen.

The inorganic substrate is preferably a porous metal substrate. Of themetals employed as the substrate, high alloys or corrosion resistantmetals and alloys, such as stainless steel, are preferred. Exemplary ofsuitable metals are nickel, the Hastalloy family of metals, the Monelfamily of metals, 316 L stainless steel and other similar alloys. Themetallic substrates are formed from metal particulate, such as metalpowder, metal fibers or combinations thereof. Preferred are substratesformed from metal fibers. Suitable for use in the invention are fibershaving diameters of less than about 0.1 to about 12 microns, preferablyabout 0.1 to about 4 microns.

Prior to coating, the metal substrates of this invention have a porerating of about 0.5 microns to about 10 microns, preferably about 2microns to about 6 microns. The void fraction of the uncoated metalsubstrate ranges from about 50 to about 95% voids, preferably about 60to about 80% voids.

The thickness of the carbon layer deposited on the porous substrateranges preferably from about 50Å to about 1000Å. While coating thesubstrate with carbon may reduce the pore rating and concomitantlyincrease the ΔP across the medium, the particular application of theassembly determines whether the ΔP adversely affects the use of theassembly.

The carbon layer is modified to present active sites which include atleast one metal species in reduced form chemically bonded to the carbonlayer. The metal species is capable of reacting or otherwise interactingwith the impurities of the gas stream in its reduced form. The term"reduced form" relates to the oxidation state of the metal in thatoxygen is chemically combined with the metal in less than thestoichiometric amount normally present in the metal oxide (i.e., themetal species is partially or substantially deoxygenated). The reducedform of the metal may consist of metal or metal suboxides chemisorbed oncarbon as well as metal-carbon intercalate compounds. In other words,the carbon layer is modified to present active sites which includepartially or substantially deoxygenated metal species that arechemically bonded to the carbon layer. Preferably, the active sitesinclude substantially deoxygenated metal species, which are capable ofreacting with trace impurities in a gas stream. Examples of metals ortheir suboxides which may be employed include manganese, magnesium, andalkali metals such as lithium, sodium and potassium.

An exemplary technique for depositing the carbon layer on the substrateis a chemical vapor deposition (CVD) technique via thedisproportionation of carbon monoxide (CO) or the dissociation of othercarbon sources such as various hydrocarbons. The deposition can occurunder various reaction times, temperatures, and gas compositions, thecontrol of which being within the skill of one in the art. For example,if CO is used as the carbon source, the disproportionation may beconducted at a temperature above about 250° C. using a gas mixture ofabout 5 to 15 percent by volume CO, 1 to 5 percent by volume hydrogenand the remainder a relatively inert gas such as nitrogen. Preferably,the disproportionation is carried out using a mixture of CO andhydrogen, which contains from about 6% to about 14% hydrogen (byvolume), at a temperature of from about 350° C. to about 450° C. Thethickness and amount of carbon deposited may be controlled by adjustingthe reaction time, temperature, gas composition and gas flow rate. Forexample, good carbon deposition may be achieved at about 435° C. on a1.1" diameter filter disk formed from sintered stainless steel fibersusing about an 88/12 (vol/vol) mixture of CO and hydrogen at a total gasflow rate through the filter disk of about 500 cc/min.

The formation of the active sites may be carried out by first depositinga precursor metal species on the carbon layer. The former may include anoxidized form of the metal and/or a deoxygenated form of the metal. Theprecursor metal species may be deposited by using a chemical vapordeposition technique, such as CVD, e.g., forming a vapor of theprecursor metal species to be deposited and contacting the layer ofcarbon deposited on the porous substrate layer with the precursor metalspecies vapor. The vapor, for example, may be generated from the metalitself or from a hydroxide or oxide of the metal. The temperature fordeposition depends on the specific porous substrate and precursor metalspecies employed and, typically, is greater than about 300° C.

As an alternate method, the precursor metal species may be depositedfrom a mixture which includes a solvent or liquid medium and theprecursor metal species, e.g., a solution or slurry of the precursormetal species in the solvent or liquid medium. Solutions ofmetal-bearing compounds, such as metal hydroxides or oxalates, may beemployed to impregnate the carbon coated substrate with the metalprecursor species. For example, if manganese is used, an aqueoussolution of manganese oxalate is suitable for such purpose.

A preferred method of depositing the precursor metal species is todissolve or form a slurry of the precursor metal species in anon-aqueous liquid medium. Preferably, the non-aqueous solvent is onewhich may be easily evaporated without leaving a residue, e.g., asolvent such as anhydrous liquid ammonia. Exemplary solutions of aprecursor metal species in a non-aqueous solvent include solutions of analkali metal, such as sodium, in anhydrous liquid ammonia. If metals,which are insoluble or only sparingly soluble in a non-aqueous solvent,such as anhydrous liquid ammonia, are to be used as the precursor metalspecies, a slurry of a very fine powder of the metal can be suitable fordepositing the metal on the carbon layer. The solution or slurry of theprecursor metal species is passed through the carbon-coated poroussubstrate, thereby depositing the precursor metal species on the carbonlayer. Nitrogen or another inert gas may be used to force the solutionor slurry through the porous substrate or suction may be used to drawthe solution or slurry through the porous substrate.

After passing the solution or slurry through the porous substrate todeposit the metal precursor species on the carbon layer, the non-aqueoussolvent may be driven off by purging with an inert gas such as nitrogen.Gradual heating to about 110° C. and, preferably 200° C. or greater, mayfacilitate the purging process.

The carbon-coated substrate bearing the deposited precursor metalspecies is then heated to form metal species chemically bonded to thecarbon layer, i.e., to form active sites on the carbon layer. The term"chemically bonded" is intended to include ionic and covalent bonds andvan der Waals forces. In other words, "chemically bonded" includesabsorbed metal and metal suboxides on carbon as well as variousmetal-carbon groups and intercalate compounds. Typically, the chemicalbonding is accomplished by heating the substrate to greater than about300° C., preferably above about 380° C. to about 400° C., and mostpreferably above about 500° C., thereby forming the active sites on thecarbon layer. The formation of the active sites may be carried out byheating the carbon-coated substrate in an inert atmosphere such as inertor noble gases such as nitrogen, argon, helium, and the like. If theactive sites include lithium, potassium or magnesium species, argon orhelium is preferably used to provide the inert atmosphere. The presenceof the inert atmosphere lessens the opportunity for impurities in theatmosphere and, in particular, oxidizing impurities, to come intocontact with the medium being activated. If impurities are present inthe gas in contact with the medium during the activation process, theimpurities may interact with and consume active sites. The loss ofactive sites may adversely affect the impurity removal capabilities ofthe reactive medium. Preferably, the activation process is carried outin an atmosphere of ultra pure inert or noble gas to avoid thedeactivation of active sites due to trace impurities in the gas. Theinert gas may also include a reducing gas, e.g., hydrogen, to facilitateactive site formation. Preferably, the inert gas employed during theactivation process includes hydrogen in a concentration of at leastabout 1 percent, more preferably from about 2 to about 35 percent, mostpreferably from about 4 to about 10 percent. In another embodiment, theactivation process may be carried out by heating the substrate togreater than about 300° C. under vacuum.

Typically, the substrate is heated to greater than about 500° C. andmaintained at that temperature for a suitable amount of time to form theactive sites (e.g., about one hour) while being purged with a mixture ofa inert gas preferably having at least about 4 percent hydrogen. Thisactivates the active sites of the carbon layer by reacting the precursormetal species with the carbon to form active sites on the substrate,i.e., to form metal species chemically bonded to the carbon layer. Themetal activated sites may exist in the reduced form (C-M), as anadsorbed metal, as a metal suboxide, or in oxidized form (CO₂ M), withthe removal of the oxygen necessary to have the preferred highlyreactive reduced form. Stated otherwise, the metal in reduced form hasno, or less than a stoichiometric amount of, oxygen, i.e., the metalspecies is partially or substantially deoxygenated. Preferably, themetal species is substantially deoxygenated. Preferably, the chemicalbond anchors the reactive metal species to the carbon and the carbon isanchored to the porous substrate. Thus, because the metal species andcarbon are anchored, contamination of the gas stream duringfiltration/purification assembly is avoided.

In another embodiment, several different metals can be depositedsequentially or as a mixture onto the carbon layer to permit removal ofselective impurities as the gas passes through each of the layers of thefilter medium. In still another embodiment, the reactive medium mayinclude two or more layers of carbon deposited on the exposed surfaces.Each carbon layer may be individually modified to present active sites.Such media may be produced by sequentially depositing a carbon layer,depositing a precursor metal species on the carbon layer and thenheating the medium to form the active sites on the carbon layer. Thissequence may be repeated until the desired number of activatedcarbon/metal layers has been produced. Reactive media, which include atleast two carbon layers with each layer presenting active sites, have anumber of advantages. Such multi-carbon layer media have a very highimpurity removal capacity. The active sites on the carbon layers mayinclude more than one metal species. For example, a reactive medium mayinclude first and second carbon layers deposited on substantially all ofthe exposed surfaces of the porous inorganic substrate. The first carbonlayer may be modified to present active sites which include at leastpartially deoxygenated first metal species chemically bonded to thefirst carbon layer and the second carbon layer may be modified topresent active sites which include at least partially deoxygenatedsecond metal species chemically bonded to the second carbon layer. Thismay permit the use of two metals which require different depositionmethods (e.g., one via CVD and the other via solution deposition).

The present reactive media are highly effective for removing a varietyof impurities from a gas. For example, a reactive medium embodying thepresent invention may be used to remove an impurity, such as oxygen,water, carbon monoxide, carbon dioxide or methane, from a gas stream,e.g., a stream of an inert or noble gas such as nitrogen or argon.

The present reactive media may also be used to remove moisture fromstreams of reactive gases such as oxygen, silanes, hydrogen chloride andhydrogen bromide. Typically, reactive media having active sites whichinclude sodium or magnesium species chemically bonded to the carbonlayer, may be employed for this purpose. Even though the reactive gasmay react with the active sites, moisture may still be chemisorbed bythe reactive medium (i.e., the water interacts with the active sites,thereby substantially removing the water from the reactive gas). Forexample, although HCl or HBr may react with an active site (e.g., activesites which include magnesium species) to form a metal halide, moisturemay still be chemisorbed by the metal halide to form a hydrate.Similarly, oxygen may react with the metal species of the active sitesto form an oxidized site. Since the oxidized sites are capable ofinteracting with moisture, the reactive media of the present inventionmay be employed to remove trace levels of moisture from a reactive gaslike oxygen. Reactive media embodying the present invention may also beused to remove oxygen from silanes.

In operation, a gas stream including impurities, is passed throughfiltration/purification assembly of the invention including the reactivefilter medium, which medium may have any one of a variety of knownconfigurations. It is recognized that a liquid stream could also befiltered/purified by altering the medium in a manner known to thoseskilled in the art. Heterogeneous particles are removed from the gasstream by well known filter mechanisms. The active sites of the mediuminteract, and preferably react, with trace homogeneous impuritiespresent in the gas stream (e.g. oxygen, carbon dioxide, carbon monoxide,water, organic compounds and the like) thereby removing the homogeneousimpurities from the gas stream. Although not wishing to be bound by anytheory, it is thought that substantially all of the impurity moleculescome in contact with the metal molecules of the reactive layer,interact, and are removed by the medium from the gas being filtered andpurified. In other words, the impurities interact with the metal specieswhich are bonded to the active sites of the carbon layer, therebyscavenging and removing the impurities from the gas phase.

Once purification is complete and all of the reduced metal is oxidizedor otherwise deactivated by contaminants, the medium may be regeneratedby heating the substrate to greater than about 300° C. in an inertatmosphere (e.g., nitrogen or argon) to reduce the oxidized metal (i.e.,regenerate the active sites). Alternatively, the regeneration may becarried out by heating the deactivated reactive medium to greater thanabout 300° C. under a hydrogen-containing atmosphere. The regenerationcapability of the reactive medium, a process which may be repeatedseveral times, provides a significant advantage over knownfiltration/purification assembly materials which typically may be usedonly once and for which there are apparently no known means ofregeneration. Regeneration is a feasible option with those embodimentsof the invention in which, as necessary, the reactivefiltration/purification assembly may be disassembled to provide accessof the media to carbon coating and to remove valve type barrier membersfrom exposure to high temperatures and contact with carbon orcarbon-producing materials.

The medium within the filtration/purification assembly of the inventionis typically regenerated after use by heating the housing portioncontaining the medium to at least about 300° C., preferably to greaterthan about 450° C., most preferably to a temperature of about 450° C. toabout 550° C., in an inert or noble gas atmosphere (e.g., nitrogen,argon, helium and the like). The carbon layer may participate inregeneration of the active sites in that, on heating in an inertatmosphere, the carbon may function as a reducing agent. Theregeneration process is typically carried out for a time period of about24 to 28 hours, although longer time periods, e.g., about 48 hours, mayalso be employed.

Alternatively, the medium within the filtration/purification assembly ofthe invention may be regenerated after use by heating the medium togreater than about 300° C., and preferably to a temperature of about450° C. to about 550° C., in the presence of a reducing atmosphere, suchas a hydrogen-containing atmosphere. For example, the medium may beregenerated by heating to greater than about 500° C. and maintaining themedium at that temperature for about one hour while it is purged with amixture of hydrogen and an inert gas. Preferably, the regenerationprocess is carried out using an atmosphere of an inert gas having fromabout 2 percent to about 100 percent hydrogen (vol:vol), more preferablyfrom about 10 percent to about 40 percent hydrogen. Typically, theregeneration of the reactive medium is carried out for a time period offrom about 8 to about 28 hours under these conditions. As with theactivation process used to originally form the active sites, theregeneration may also be carried out under an inert gas containing alesser amount of hydrogen. With lower concentrations of hydrogen, longertimes and/or higher temperatures are typically employed for theregeneration process. For example, the regeneration of a stainless steelreactive medium may be carried out by heating the medium for 12 to 24hours at about 500° C. while purging with a 35:65 (vol:vol) mixture ofhydrogen and nitrogen.

Assembly of the device according to the present invention may beaccomplished by various methods. The manner in which the devices of thepresent invention are assembled depend, in part, on the types of barriermembers employed and the particular configuration of the housing. Inmost embodiments of the invention it is preferred that the individualcomponents, i.e., reactive filter, barrier members, etc., be located inthe appropriate section of the housing and, as appropriate, thereaftertreated to put the active carbon and metal coatings thereon. In thoseinstances in which another type of barrier member is employed, such as avalve, the device is partially assembled with the housing portioncontaining the metal filters coated with a carbon layer and containingactive metal species formed separately from the housing portioncontaining a valve and the two housing sections are thereafter joined.The exact order in which the barrier member(s) and reactive filtermedium are joined to or within the housing section(s) depends in part onthe manufacturing and assembly facilities. Typically, the downstreambarrier member, such as a small reactive type filter is fixedlypositioned in the outlet which communicates with the housing, transverseto the downstream opening of the outlet such that all fluid passing outof the housing during operation, or into the downstream side of thehousing immediately prior to placing the filter unit into service, mustpass through the reactive type filter.

The manner of permanently locating the reactive type filter in theoutlet could be accomplished by swaging, staking, brazing, sintering orpress-fitting of the barrier member at the appropriate location.

Thus, by way of example, in the embodiments illustrated in FIGS. 2 and3, a metal filter element, such as a precursor substrate to a reactivefilter, i.e., prior to carbon coating and bonding of metal species, maybe located, fixedly or removably, at the end of the downstream outlet19. The outlet may be formed as part of a downstream housing section 15aor 15a' or may be attached to section 15a or 15a' by methods such aswelding, either before or after the downstream barrier member is fixedlypositioned in the outlet. The principal reactive metal filter which isthe precursor or substrate prior to coating to form the fluid permeable,reactive medium filter 13 (in FIGS. 2 and 3) may then be located in thehousing by removably or fixedly securing the metal filter substrate toeither or each of housing sections 15b (or 15b') and 15a (or 15a'), withor without the outlet 19 joined thereto. Typically housing element 15b(or 15b'), reactive filter medium 13 and housing member 15a (or 15a')are joined by welding, brazing or similar technique. In the embodimentillustrated in FIG. 3, the metal filter substrate, which ultimatelybecomes a barrier member 21', which is the same as or substantiallysimilar to the downstream barrier member 23 or 23', is fixedly orremovably positioned in the inlet 17'. This is done preferably at theentrance or upstream end of the inlet 17' in the same manner as is doneat the outlet 19. As with the outlet 19 in housing section 15a, theinlet 17' and housing section 15b' may be attached to one another eitherbefore or after the metal filter substrate which forms reactive mediumfilter 21' is fixedly positioned in the inlet 17'.

In the embodiment of the type illustrated in FIG. 3, in some instancesthe upstream housing section(s), inlet and filter member may have asimilar or identical configuration to the downstream housing section(s),outlet and filter member. Costs of manufacturing can be saved when theupstream and downstream portions of the assembly are identical.

The metal filter substrates, positioned within the designated portionsof the assembly, may be more efficiently coated with carbon andactivated with metal species by positioning the metal filter substratesin the appropriate housing section(s) prior to the treatment steps andthereafter assembling the separate housing sections. Thus, in theembodiment illustrated in FIG. 2, the outlet 19 and housing sections 15aand 15b, containing the metal filter substrate which forms the reactivefilter medium 13, are separately assembled and treated to produce carboncoatings bonded to active metal species prior to joining the upstreamhousing section 15c and the inlet 17 containing the poppet valve 21. Theupstream housing section 15c may be joined by welding or the like tohousing section 15b. A similar procedure may be used with the embodimentof FIG. 3.

Coating of the metal filters and activation with metal species to formthe reactive filter medium and barrier member(s) of the reactive filtertype may be performed in the manner indicated above and in U.S. Pat. No.5,637,544. Alternatively, the following modified procedure may beemployed. The filtration/purification assembly or portion thereofcontaining metal filter substrate(s) may be initially subjected to atreatment with a reducing agent. Thus, the portion of the filterassembly containing the metal filter substrates may be connected to amanifold which is also in fluid communication with sources of an inertgas or a noble gas, such as argon, helium and in some instancesnitrogen, as well as hydrogen and carbon monoxide. Initially, to purgeair and atmospheric contamination from the assembly, the inert gas, suchas argon, is supplied to the housing at a rate of 0.7 to 1.2 cc/min-mm²,typically at or slightly above ambient temperature. Thereafter, in orderto strip the surface of the naturally occurring mixed oxides present onthe surface of metals, such as iron and chromium oxides, typicallypresent on a stainless steel surface, and form a reduced metal surface,argon flow is terminated and the system is purged with purified hydrogenat a rate of about 0.8 to about 1.5 cc/min-mm². The purging is continuedat ambient temperature for approximately 10 to 20 minutes or untilhydrogen completely displaces argon from the system. The temperature isthen increased to about 400 to about 450° C. at a rate of about 2 to 8C.°/min. Hydrogen flow is allowed to continue as the filter ismaintained within this temperature range for about 3 hours to from auniformly reduced or elemental metal surface. Thereafter, an inert or anoble gas, such as 100% argon, is passed through thefiltration/purification assembly at a flow rate of about 2.2 to about2.6 cc/min-mm². Argon flow is continued through the assembly and thetemperature is permitted to drop to about 300 to about 350° C.

A controlled oxidation step is then conducted which renders a relativelythick and uniform oxide on the surface of the metal which consistsprimarily of iron oxide when a metal such as stainless steel isemployed. This uniform iron oxide layer acts as a catalyst to promoteuniform dissociation of carbon monoxide and deposition of elementalcarbon on the substrate surface. This also has the effect of increasingthe number of active metal sites on the carbon of the reactive filterelement. The oxygen is introduced to the assembly to provide an oxygenand argon mixture containing about 35% to about 45%, preferably about40% oxygen with a flow rate of about 2.2 to about 2.6 cc/min-mm². Flowof this oxygen/argon mixture is continued within the range of about 300to about 350° c. for a period of about 2 to about 4 hours, preferablyabout 2.5 hours. Thereafter, the furnace in which the reactive filterelement is heated is turned off and 100% argon is passed through theassembly at a flow rate of about 2.2 to about 2.6 cc/min-mm² for a timesufficient to allow the uniformly oxidized substrate to cool to or closeto room temperature if carbon coating is not begun immediatelythereafter. Subsequently, as described above, the oxidized metal filtersubstrate is coated with carbon and active metal species are introducedto the carbon with a vapor deposition process or a solvent-metalprocedure as described above.

As a result of the procedure of reducing and subsequently providing auniform oxidized surface to the metal filter substrate or element, theratio of the total surface area of the carbon coating to the totalsurface area of the metal filter substrate is about 25 to 65 m² /m² ascompared to 2.3 to 3.6 m² /m² without oxidation pretreatment. This alsoproduces a ratio of sodium metal coating (when the carbon is activatedwith sodium metal species) to surface area of carbon coating of about1.2 to about 1.6 mg Na/m² carbon. The resultant oxygen capacity is about4 to about 6 times greater than without oxidation pretreatment.

EXAMPLE

An example of a specific method of preparing the housing or housingportions containing metal filter substrates prior to coating with carbonand bonding metal species to active sites on the carbon, such that themetal substrates have a uniform oxide coating may be performed asfollows. A housing section containing a metal filter substrate welded inplace across a fluid flow path was attached to a manifold that was inturn connected to sources of several different pure gases. The housingsection was supported within a furnace and was initially purged ofatmospheric contamination with purified argon at a flow rate of 500cc/min for approximately 10 minutes at ambient temperature. Thereafter,the flow of argon through the housing section was discontinued and thehousing section was purged with purified hydrogen at a flow rate of 600cc/min, also at ambient temperature, for a period of about 10 to 20minutes. The temperature of the housing section containing the metalsubstrate was gradually increased to 425° C. at a rate of 2 C.°/min. Thehousing section was maintained, with hydrogen flowing at a rate of 600cc/min, at 425° C. for three hours. Thereafter, hydrogen flow wasdiscontinued and pure argon was passed through the housing section at aflow rate of 1500 cc/min. With argon flowing through the housing sectionat the same rate, the temperature of the housing section was allowed todecrease to 305° C. Thereafter, oxygen was mixed with argon to provide agaseous mixture flowing through the housing section of 40% oxygen and60% argon at a flow rate of approximately 1500 cc/min. This wascontinued for a period of 2.5 hours at a temperature of 305° C.Thereafter, the furnace was turned off and 100% argon was passed throughthe housing section at a rate of 1500 cc/min. Carbon deposition andactivation of sites on the carbon deposit were performed as indicatedabove.

What is claimed is:
 1. A reactive medium for purifying a fluidcomprising:a porous metal substrate; a layer including carbon; an oxidelayer between the porous metal substrate and the layer including carbon;and active sites including at least partially deoxygenated metalspecies, the active sites being bonded to the layer including carbon. 2.The reactive medium of claim 1 wherein the oxide layer comprisesoxidized metal of the substrate.
 3. The reactive medium of claim 2wherein the oxide layer is disposed on substantially all surfaces of theporous metal substrate.
 4. The reactive medium of claim 3 wherein theoxide layer comprises iron oxide.
 5. The reactive medium of claim 4wherein the metal comprises stainless steel.
 6. The reactive medium ofclaim 1, wherein the oxide layer is disposed on substantially allsurfaces of the porous metal substrate.
 7. The reactive medium of claim1 wherein the oxide layer is uniformly disposed between the porous metalsubstrate and the carbon layer.
 8. The reactive medium of claim 1wherein the ratio of the total surface area of the carbon coating to thetotal surface area of the metal substrate is in the range from about 25to about 65 m² /m².
 9. The reactive medium of claim 1 wherein thepartially deoxygenated metal species is selected from the groupconsisting of alkali metals, manganese, magnesium, and mixtures thereof.10. The reactive medium of claim 9 wherein at least one of thedeoxygenated metal species includes sodium.
 11. The reactive medium ofclaim 10 wherein the ratio of sodium metal to surface area of carbon isthe range from about 1.2 to about 1.6 mg sodium/m² carbon.
 12. Thereactive medium of claim 1 wherein the metal of the porous metalsubstrate is selected from the group consisting of nickel, the Hastalloyfamily of metals, the Monel family of metals, and stainless steel.
 13. Areactive medium for purifying a fluid comprising:a porous metalsubstrate; an oxide layer disposed on substantially all surfaces of theporous metal substrate; at least one layer of carbon disposed on theoxide layer; and active sites including at least partially deoxygenatedmetal species, the active sites being bonded to the carbon layer andincluding at least one of manganese, magnesium, and an alkali metal. 14.The reactive medium of claim 13 wherein the metal of the porous metalsubstrate is selected from the group consisting of nickel, the Hastalloyfamily of metals, the Monel family of metals, and stainless steel, andwherein the oxide layer comprises oxidized metal of the substrate. 15.The reactive medium of claim 13 wherein the porous metal substratecomprises stainless steel and the oxide layer comprises iron oxide. 16.The reactive medium of claim 15 wherein the iron oxide layer is disposedon substantially all surfaces of the stainless steel substrate.
 17. Areactive medium for purifying a fluid comprising:a porous stainlesssteel substrate; a layer including carbon; an oxide layer between theporous stainless steel substrate and the layer including carbon, theoxide layer comprising iron oxide and being disposed substantially onall surfaces of the porous metal substrate; and active sites includingat least partially deoxygenated metal species, the active sites beingbonded to the layer including carbon.
 18. The reactive medium of claim17 wherein the partially deoxygenated metal species is selected from thegroup consisting of alkaline metals, manganese, and magnesium, andmixtures thereof.